I have a 1992 Cadillac Deville I picked up. Owner thought the head gasket was blown. OIl checked clean as well as the radiator fluid. Car runs real rough unless you rev it up and then its clouds of black smoke. Seems to be using an awful lot of gas also. Checked the codes and I have a Code 52 and 91. Not sure where to go next on this thing, if anyone has any help please chime in!